The conventional method for providing maneuvering control of ships is by use of a rudder mounted vertically at the aft end of the ship. However, when it is attempted to adapt such conventional control systems to small waterplane-area twin hull design watercraft, major structural design problems became evident.
By way of background, semisubmerged or Small Waterplane Area Twin Hull ships, sometimes referred to as SWATH ships, have been developed for high-speed operation at high sea states. THE SWATH ship consists of three major parts: (1) lower hulls which provide the majority of the buoyancy, (2) struts (either single or tandem) that are the vertical structural members that provide the remaining required buoyancy and the necessary ship hydrostatic properties, and (3) an upper cross structure that ties the pair of struts and lower hull assemblies together and provides a working platform. U.S. Pat. Nos. 3,623,444 and 3,897,744 issued to Thomas G. Lang disclose ships of this configuration which have better operational characteristics than conventional ships and can operate at much higher sea states.
As pointed out in U.S. Pat. No. 3,623,444, the effect of surface wave drag on the stability and maximum speed of seagoing vessels is well known to marine designers. Increased sea states magnify the stability and speed problems of vessels due to the inherent pitch, heave and roll tendencies of such vessels.
The above-noted patents to Lang point out a number of configurations for vessels to be operated at high speed under adverse sea conditions. All of these configurations include a pair of lower hulls, an upper cross structure and struts for attaching the upper cross structure to the lower hulls. But, as pointed out by these patents, the mere configuring of a vessel with these components does not solve the speed and stability problems.
The vessel must be hydrodynamically balanced to stabilly operate at high speed in increased sea states.
Optimum or near optimum strut/lower hull forms result when the lower hulls extend aft of the struts. This configuration, although advantageous from a seakeeping, hydrodynamic and power requirement viewpoint presents severe problems for application of conventional ship rudder maneuvering system. The lack of structural support for the rudder has resulted in configuration compromises that shorten the lower hull relative to the strut, which in turn dictates that the rudder be supported from the rear strut. Other approaches include ring rudders, rudders mounted in the prop wash that are supported from the lower hull and strut trailing edge flaps.